SYSCTL.D(5) sysctl.d SYSCTL.D(5)
NAME
sysctl.d - Configure kernel parameters at boot
SYNOPSIS
/etc/sysctl.d/*.conf
/run/sysctl.d/*.conf
/usr/lib/sysctl.d/*.conf
key.name.under.proc.sys = some value
key/name/under/proc/sys = some value
key/middle.part.with.dots/foo = 123
key.middle/part/with/dots.foo = 123
-key.that.will.not.fail = value
key.pattern.*.with.glob = whatever
-key.pattern.excluded.with.glob
key.pattern.overriden.with.glob = custom
DESCRIPTION
At boot, systemd-sysctl.service(8) reads configuration files from the
above directories to configure sysctl(8) kernel parameters.
CONFIGURATION FORMAT
The configuration files contain a list of variable assignments,
separated by newlines. Empty lines and lines whose first non-whitespace
character is "#" or ";" are ignored.
Note that either "/" or "." may be used as separators within sysctl
variable names. If the first separator is a slash, remaining slashes
and dots are left intact. If the first separator is a dot, dots and
slashes are interchanged. "kernel.domainname=foo" and
"kernel/domainname=foo" are equivalent and will cause "foo" to be
written to /proc/sys/kernel/domainname. Either
"net.ipv4.conf.enp3s0/200.forwarding" or
"net/ipv4/conf/enp3s0.200/forwarding" may be used to refer to
/proc/sys/net/ipv4/conf/enp3s0.200/forwarding. A glob glob(7) pattern
may be used to write the same value to all matching keys. Keys for
which an explicit pattern exists will be excluded from any glob
matching. In addition, a key may be explicitly excluded from being set
by any matching glob patterns by specifying the key name prefixed with
a "-" character and not followed by "=", see SYNOPSIS.
Any access permission errors and attempts to write variables not
present on the local system are logged, but do not cause the service to
fail. Debug log level is used, which means that the message will not
show up at all by default. Moreover, if a variable assignment is
prefixed with a single "-" character, any failure to set the variable
will be logged at debug level, but will not cause the service to fail.
All other errors when setting variables are logged with higher priority
and cause the service to return failure at the end (other variables are
still processed).
The settings configured with sysctl.d files will be applied early on
boot. The network interface-specific options will also be applied
individually for each network interface as it shows up in the system.
(More specifically, net.ipv4.conf.*, net.ipv6.conf.*, net.ipv4.neigh.*
and net.ipv6.neigh.*).
Many sysctl parameters only become available when certain kernel
modules are loaded. Modules are usually loaded on demand, e.g. when
certain hardware is plugged in or network brought up. This means that
systemd-sysctl.service(8) which runs during early boot will not
configure such parameters if they become available after it has run. To
set such parameters, it is recommended to add an udev(7) rule to set
those parameters when they become available. Alternatively, a slightly
simpler and less efficient option is to add the module to modules-
load.d(5), causing it to be loaded statically before sysctl settings
are applied (see example below).
CONFIGURATION DIRECTORIES AND PRECEDENCE
Configuration files are read from directories in /etc/, /run/,
/usr/local/lib/, and /lib/, in order of precedence, as listed in the
SYNOPSIS section above. Files must have the the ".conf" extension.
Files in /etc/ override files with the same name in /run/,
/usr/local/lib/, and /lib/. Files in /run/ override files with the same
name under /usr/.
All configuration files are sorted by their filename in lexicographic
order, regardless of which of the directories they reside in. If
multiple files specify the same option, the entry in the file with the
lexicographically latest name will take precedence. Thus, the
configuration in a certain file may either be replaced completely (by
placing a file with the same name in a directory with higher priority),
or individual settings might be changed (by specifying additional
settings in a file with a different name that is ordered later).
Packages should install their configuration files in /usr/lib/
(distribution packages) or /usr/local/lib/ (local installs). Files in
/etc/ are reserved for the local administrator, who may use this logic
to override the configuration files installed by vendor packages. It is
recommended to prefix all filenames with a two-digit number and a dash,
to simplify the ordering of the files.
If the administrator wants to disable a configuration file supplied by
the vendor, the recommended way is to place a symlink to /dev/null in
the configuration directory in /etc/, with the same filename as the
vendor configuration file. If the vendor configuration file is included
in the initrd image, the image has to be regenerated.
EXAMPLES
Example 1. Set kernel YP domain name
/etc/sysctl.d/domain-name.conf:
kernel.domainname=example.com
Example 2. Apply settings available only when a certain module is
loaded (method one)
/etc/udev/rules.d/99-bridge.rules:
ACTION=="add", SUBSYSTEM=="module", KERNEL=="br_netfilter", \
RUN+="/lib/systemd/systemd-sysctl --prefix=/net/bridge"
/etc/sysctl.d/bridge.conf:
net.bridge.bridge-nf-call-ip6tables = 0
net.bridge.bridge-nf-call-iptables = 0
net.bridge.bridge-nf-call-arptables = 0
This method applies settings when the module is loaded. Please note
that, unless the br_netfilter module is loaded, bridged packets will
not be filtered by Netfilter (starting with kernel 3.18), so simply not
loading the module is sufficient to avoid filtering.
Example 3. Apply settings available only when a certain module is
loaded (method two)
/etc/modules-load.d/bridge.conf:
br_netfilter
/etc/sysctl.d/bridge.conf:
net.bridge.bridge-nf-call-ip6tables = 0
net.bridge.bridge-nf-call-iptables = 0
net.bridge.bridge-nf-call-arptables = 0
This method forces the module to be always loaded. Please note that,
unless the br_netfilter module is loaded, bridged packets will not be
filtered with Netfilter (starting with kernel 3.18), so simply not
loading the module is sufficient to avoid filtering.
Example 4. Set network routing properties for all interfaces
/etc/sysctl.d/20-rp_filter.conf:
net.ipv4.conf.default.rp_filter = 2
net.ipv4.conf.*.rp_filter = 2
-net.ipv4.conf.all.rp_filter
net.ipv4.conf.hub0.rp_filter = 1
The rp_filter key will be set to "2" for all interfaces, except "hub0".
We set net.ipv4.conf.default.rp_filter first, so any interfaces which
are added later will get this value (this also covers any interfaces
detected while we're running). The glob matches any interfaces which
were detected earlier. The glob will also match
net.ipv4.conf.all.rp_filter, which we don't want to set at all, so it
is explicitly excluded. And "hub0" is excluded from the glob because it
has an explicit setting.
SEE ALSO
systemd(1), systemd-sysctl.service(8), systemd-delta(1), sysctl(8),
sysctl.conf(5), modprobe(8)
systemd 245 SYSCTL.D(5)